A deal would see much of the already small private energy market on this side of the border also coming under State control.

A joint venture between the State-controlled Ireland Strategic Investment Fund (ISIF) and funds controlled by Goldman Sachs is understood to have submitted a final round bid to buy power firm Viridian for up to €1bn.

If the joint venture is successful, it will cement the State's effective control of the bulk of Ireland's power generation and supply, as well as electricity and gas distribution.

Final binding bids in the Viridian sales process were due yesterday. It's believed the joint venture between ISIF and Goldman Sachs is among the final offers submitted. Viridian trades as Energia in the Republic of Ireland.

It is being sold by Arcapita, the Bahrain private equity fund. It put the power firm up for sale last year. In February, UK utility giant Centrica, which owns Bord Gáis Energy, failed to make it to the second round of bidding. State-owned ESB, which is already the biggest energy business on the island of Ireland did not bid for Viridian. That is partly because it is focused on building up capacity outside Ireland in preparation for the advent of a single European energy market.

It's believed that just a handful of bidders now remain in the final round of contenders to buy Viridian, but it's unclear when a decision on the winning bid will be made.

While there have been expectations that Virdian could sell for about €1bn, it's been speculated that the final selling price could come in below that.

In Northern Ireland, Viridian owns Power NI, which it supplies electricity to about 610,000 homes and businesses.

In the Republic, Viridian owns Energia, which supplies electricity to the domestic and commercial market.

A sale would include two power stations in Dublin and an extensive windfarm portfolio.

Power NI's former sister company Northern Ireland Electricity Networks Limited (NIE Networks), which owns the electricity transmission infrastructure network in the North is owned by the ESB.

Viridian's power generation and power supply businesses could be broken up after a sale, especially if the business is bought by a consortium.

Viridian's group pro-forma earnings before interest, tax, depreciation and amortisation (EBITDA) were £97.5m in its last financial year, down from £99m a year earlier.

Its assets include 747 megawatt Huntstown power plants in Dublin, as well as 300 megawatts of windfarms that are already operational or being developed.

The National Treasury Management Agency (NTMA) controls and manages the Ireland Strategic Investment Fund (ISIF), which was established in 2014 with a statutory mandate to invest on a commercial basis in a manner designed to support economic activity and employment in the State.

The NTMA points out that assets are managed at arms length, and co-investment is a key element of ISIF's strategy.

It actively seeks to attract partners to invest alongside it, thereby enabling the fund to leverage its resources and maximise economic impact for Ireland.

By the end of last December, ISIF had committed €2bn to Irish investments.

When combined with funds from co-investment partners, a total of €4.9bn had been committed to Irish projects.

Among ISIF's investments is a 15pc stake in Irish stockmarket-listed life sciences investment firm Malin, which was co-founded by former Elan chief executive Kelly Martin.

It is also involved in residential and commercial property development investments.